High EGT's at full throttle

·         John Dilks

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Hi Doug,

I've got an old 3300 (serial no 33A163) in a Sonex.

The engine was second hand and has approx. 80hrs on it, about 25hrs after a strip and rebuild prior to using it in the sonex.

Since running in the sonex it's been fine at cruise power (CHT's 120-140C & EGT's 700C) but at full power the EGT's are up around 750C or higher (the CHT's don't seem to get too high, max 160C) and get a bit of rough running. I'm guessing this is detonation so throttle back if it occurs which cures it.

I checked the carb and all seemed fine apart from the needle didn't appear to be the same shape as the one in the manual, ref pictures attached - Needle picture 1 = what I expected, Needle picture 2 = what is installed in the carby.

I ordered a new needle from the UK Jabiru distributer for a hydraulic lifter 3300 but it's turned up and looks pretty much like needle 2 (only difference I can tell is the existing needle has 2 smaller grooves below the bottom circlip groove compared to 1 groove in the new needle - both have 4 circlip grooves).

Hope you can let me know what needle I should be using!

NB Climb out (full power) RPM is around 2800, usually cruise around 2450. Also the needle was on the bottom circlip groove.

Cheers
John

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·         Doug Smith

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Hi John,
The photos don't appear to have made it...

The Jabiru needles should only have 1 circlip groove, but they have additional identification grooves on there.

The egts are certainly getting up there, but not radically so. The rough running probably isn't caused by outright leanness, I'm guessing there's an uneven mixing element to it as well. What are you running for engine air intake to the cowl and for the duct between the filter and the carby? Are you running a muffler or straight pipes? The other thought is fuel flow... have you confirmed your fuel system is delivering fuel at the rate needed for full power? There's detail on how to check that in the installation manual, just make sure you do it with the carb hooked up as the needle seat can be a significant restriction.

Cheers!
Doug

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·         Rob Turk

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Couple of suggestions:
1. Your full power rpm is a bit low. You may still be in the transition area on the needle, which means you don't get full fuel from the main jet. Could you be running a prop that's a bit course?
2. Have you checked the main jet size? There's been various combinations of main and needle jets with different shape needles. Since you have an early engine you may also have an early set of jets. If you run a small main jet, you could see the same thing. The Jabiru overhaul manual has a table of matching parts which are a good start, but individual installations may require that you use a somewhat larger main jet. Sky-Craft can provide you with larger main jets.
3. Have you checked the fuel delivery capability of your fuel system? For a 3300 the system should deliver at least 40-45l/minute. To test, take the fuel line off the carby and measure the time to fill a 1l bottle.

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·         John Dilks

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Hi Doug & Rob,

Thanks for your reply's.

I had quick chat with Dave from Sky-Craft and the needle I ordered was a for an hydraulic lifter 3300 which runs a little leaner than the earlier 3300. I returned it and he sent me the recommended one which looks identical to the one in the carby now - so it looks like I have the correct one for the engine.

In answer to Rob's comments;
1. The sonex prop is course, as I remember 54dia x 62 pitch, a couple of the other UK sonex's with the same prop did report higher take of RPM, can't remember the exact figures.

2. The main jet is 285, did try a 290, that brought the EGT's down about 20 - 30C throughout but exhaust (and underside of fuse!) very sooty.

3. Checked fuel flow as part of the final inspection, as I remember OK per Jab manual, over 45l/hr.

Also I have a jab muffler (old design with two inlets & two outlets).

One thing that I thought it may be is the air intake is under the cowl, i.e. drawing in warm air that's past the cylinders. Also the exhaust muffler in the same bit of space won't help. This is pretty much the same set up as the other UK sonex's which don't appear to have this issue but I may have been too careful with the baffling and no air is getting around the baffles (and hence not going past the cylinders) into the back of the engine to keep the intake air a little cooler??

Cheers
John

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·         Doug Smith

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Hi John,
First off, it does seem that the aircraft is a little over-propped at the moment... BUT that might be more a issue of the engine not making power rather than the prop being too coarse. Remember that the older 3300 engines had a little less power than the modern units so, depending how much of a rebuild you gave it, you might be a handful of horses short. That means that a prop that works OK on a new engine might be a little greedy for you. I don't think that's the cause of your issues but just something to keep in mind.

Drawing engine air from inside the cowls can be tricky. The Bing carby likes smooth, straight airflow. Turbulence, uneven flow (drawing preferentially from one side or the other), pressure differences and air temp can all upset it. Do you have anything at all attached to the carby inlet or does the carb just suck from that space? I'd recommend a bell-mouth (intake trumpet) of some sort at the very least... possibly a pod filter would work well to minimise turbulence at the carby inlet? You also have to make sure the sense port is connected to somewhere where it measures pressure similar to that seen on the carby intake.

Have you done a manometer check of the cowl pressures? It would be interesting to see if the cowls are pressurising at speed: that pressure could throw the carb mixtures off too.

Regards,
Doug.

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·         John Dilks

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Hi Doug,

Thanks for the further thoughts. I currently have an automotive conical foam filter on a L-bend on the carby, so may be a few issues with the airflow - I did try a larger K&N filter with an internal bell-mouth but there's not much space in the sonex cowls and it wouldn't fit! I'll try a smaller / different shaped one with a straight connector if I can find one that fits.

When I was attempting to fit the K&N filter I noticed the muffler had been blowing down the rear seam. After attempting to have it welded (it cracked) I've fitted straight through pipes which are per the Sonex Jabiru US installation standard and are used on a couple of Sonex's flying over here. This has improved things a lot but still not 100% free from rough running.

I haven't done a manometer check on the cowls, didn't fancy drill holes in them after painting them!

One thing I've noticed is it appears to run relatively cool EGT's at cruise power settings with slightly sooty exhaust so was wondering if being an early 3300 I should be using a different needle to later engines, is there anyone I can contact directly at Jabiru to find out if I can get hold of an early spec needle?

Thanks for your help,
John

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·         Doug Smith

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Hi John,
Some of those pod filters can put a lot of swirl into the airflow... Can you point the "L" the other way and see if that makes any difference (i.e. move the filter inlet from the left to the right of the aircraft [or vice versa])?

In theory the pressure tappings for the manometer just have to come from inside the ducts somewhere... you can thread a piece of 1/4" ally pipe in there from underneath if you don't want to disturb the paintwork 🙂

It does sound like you've got a fairly unique installation there... The gents at @jabiru.net.au">engines@jabiru.net.au should be able to help you out.

Regards,
Doug.